I freaked out once when I worked at an aquarium and I broke a BIG mercury thermometer. But I learned that my managers broke several - in tanks - and I don't believe there were too many issues from what I could tell. And learned that my parents, in the old days, used to play with it. Its cool stuff...and they are still alive, BTW
So don't eat it, siphon it out or use an eyedropper to target it specifically.
There are several saltwater fish thought to have high levels of mercury - including many in the jack family (tuna, mackerel). However, if I recall they also compared the tissue to fish collected a hundred years ago and that are in museum collections...and the levels were basically similar. So whether it is a poisoning issue due to human polluting or certain fish for whatever reason have higher levels of mercury, it is a bit tought to tell. There are some animals that concentrate what we would consider "toxins" without much trouble.